More than 100 children are trapped in a building which finds itself under heavy attack in east Aleppo in Syria, the United Nations Children's Fund, UNICEF has warned Tuesday.

"As violence continues to escalate in Aleppo today, thousands of children are suffering in silence, and coming under brutal attack as the world watches. It is time for the world to stand up for the children of Aleppo and bring their living nightmare to an end," UNICEF said in a statement.

"According to alarming reports from a doctor in the city, many children, possibly more than 100, unaccompanied or separated from their families, are trapped in a building, under heavy attack in east Aleppo," UNICEF said.

UNICEF told Anadolu Agency in an email: "We are unable to confirm location as we don’t want to expose the children to more danger."

Urging all parties to allow the safe and immediate evacuation of all children, the agency added: "UNICEF is deeply concerned by unverified reports of extra judicial killings of civilians including children and reminds all parties of their responsibilities under international law."

According to the UN, the Syrian regime and its allies entered homes and shot dozens of civilians dead during its offensive in eastern Aleppo on Monday.

UNICEF called on "all warring parties to immediately reach a truce in Aleppo and allow humanitarian organizations to deliver urgent assistance to families and children in need wherever they are, and without conditions."

Over the course of the last 27 days, an estimated 990 civilians have been killed in eastern Aleppo in attacks by the Syrian regime and allied militias, according to local source-based reports from the city.